Fluidized bed reactors are effective means for generating heat and, in various forms, can carry out the processes of drying, roasting, calcining, heat treatment of solids with gases in the chemical, metallurgical, and other material processing fields, and the generation of hot gases, including steam, for use in driving electric power generation equipment, for process heat, for space heating or for other purposes. In a reactor generating hot gases, air is passed through a bed of particulate material which includes a mixture of inert material and a fuel material such as coal, wood waste or other combustible materials. Where the combustion of bituminous or anthracite coal or other fuels containing a high sulfur component is undertaken, a material such as lime or limestone which will react with the sulfur released by combustion may be provided in the bed.
Fluidized bed technology has been widely applied to accomplish a variety of chemical reactions, heating, cooling and other processes for the past few decades. In the United States fluidized beds were first used as a combustion technique starting in the late 1960's and early 1970's. Initially, bubbling fluidized beds were the preferred technology; however, gradually the emphasis was shifted to circulating fluidized beds. Bubbling fluidized beds operate at lower superficial velocities than circulating fluidized beds, usually 3-6 feet per second verses 18-22 feet per second, respectively. Superficial velocities refer to the velocity of the products of combustion in the reaction chamber just above the dense bed area.
The use of fluidization as a combustion technique is accomplished by having the chemical reactions of combustion take place in a bed of granular material which has been suspended, or lifted, or fluidized by all, or part of the combustion air. If the mean particle size of the granular material is sufficiently small and/or if the velocity of the combustion air is high enough, very high quantities of the bed material are entrained, or elutriated by the products of combustion and there is virtually no discernable level of top-of-bed.
A typical bubbling fluidized bed boiler system is described in U.S. Pat. No. 4,301,771 (Jukkola et al.), which issued on Nov. 24, 1981. The reaction chamber consists of a bubbling bed in the lower section and a freeboard in the upper section, all encased in a watercooled membrane wall. The membrane wall may provide a part or all of the required heat transfer surface area for heat recovery. Additional heat transfer surface area, if necessary, can be provided by in-bed tubes.
Circulating fluidized bed boiler or reactor systems involve a two phase gas-solids process which promotes solids entrainment within the upflowing gas stream in the reaction chamber and then recycles the solids back into the reaction chamber with a high rate of solids circulation. The rate of solids circulation in the circulating fluidized bed process is about fifty times that of a bubbling bed process. Moreover, circulating fluidized bed systems typically use elongated reaction chambers which increase solids residence time, thus increasing carbon combustion efficiency, increasing heat transfer, and decreasing carbon monoxide emission levels.
Circulating fluidized bed boilers produce both a dense bed or "bubbling" bed and a dilute phase or "fast" bed. The bubbling bed is at the bottom of the reaction chamber with the dilute phase above. The dilute phase will typically have solid loadings of 3 to 9 pounds per pound of gas. Operation with both a dense and dilute phase is achieved by permitting some of the combustion air to bypass the dense bed and enter at the bottom of the dilute phase. The dilute phase gives very good turbulence and mixing with no streamline or laminar flow. The "slip" between velocities of the entrained solids and the flue gas is quite large and this gives good solids "fallback" or "back-mixing".
Various examples of known circulating fluid bed systems are described in U.S. Pat. Nos. 4,1656,717 (Reh et al.), which issued on Aug. 28, 1979, and 3,625,164 (Spector), which issued on Dec. 7, 1971, and an article by A. M. Leon and D. E. McCoy, "Archer Daniels Midland (ADM) Conversion to Coal," Circulating Fluidized Bed Technology, Proceedings of the First International Conference on Circulating Fluidized Beds, Pergamon Press, Nov. 18-20, 1985, pp. 341-348.
FIG. 1, attached hereto, demonstrates one conventional circulating fluidized bed boiler system contemplated herein. FIG. 1 is a schematic representation of a circulating fluidized bed steam generator system comprising a reaction chamber 1 formed by a disc-type seal-welded membrane waterwall 2, all of which is encased in a metal frame or housing 3. A distribution plate 4 is disposed at the bottom of reaction chamber 1 wherein primary air 5 is introduced to the lower portion of reaction chamber 1 via a windbox 6 and distributed via constriction plate or distributor 4 together with tuyeres 8.
Windbox 6 is an air chamber encased by seal-welded waterwalls 2 which are extensions from the waterwall forming reaction chamber 1. Disposed at the lower portion of reactor 1 is a refractory material 7 used to protect the membrane waterwall from erosion due to the high turbulence in the dense bed. Air from windbox 6 is introduced to the lower portion of reaction chamber 1 via tuyere 8. Secondary air is introduced via secondary air inlets 9 which are located above the recycle port 10. Optionally, secondary air may also be introduced through lower secondary air inlets 11 which may be located on or near the same plane as recycle port 10.
Water is introduced to membrane waterwalls 2 and heat exchange tubes 12 via water drum 13. Carbonaceous materials, such as coal, wood, petroleum coke or the like, and a desulfurizing agent, such as limestone, are introduced into reaction chamber 1 via feed conduit 14. Carbonaceous material and desulfurizing agent are usually introduced to the lower portion or dense bed of reaction chamber 1. Prior to the introduction of the carbonaceous material start-up burner 25 is ignited to bring the temperature within reaction chamber 1 up to operating conditions.
Thereafter, primary air 5 is introduced via windbox 6 and tuyeres 8 for fluidizing the carbonaceous material. Simultaneously, burners 25 are used to ignite the carbonaceous material as it moves through reaction chamber 1 in contact with oxygen-containing fluidizing gas. The primary air is usually insufficient to burn all of the incoming fuel completely and creates a substoichiometric condition, which deliberately induces an incomplete combustion process under a reducing atmosphere. This is deliberately done in an attempt to limit oxidation of devolatized fuel nitrogen. Devolatized fuel nitrogen may be partially oxidized upon coming into contact with oxygen. However, under a reducing atmosphere which is rich in carbon and carbon monoxide, substantial quantities of the oxidized nitrogen oxide would be reduced to elemental nitrogen. The result is low nitrogen oxide emissions, which is often necessary under air pollution regulations.
As the gas stream leaves the dense bed it carries incomplete combustion product with it. At this junction secondary air is introduced via inlets 9, and optionally through inlets 11, in sufficient quantity to complete combustion of the carbonaceous material. Moreover, unburned carbon or carbon monoxide is subjected to an ample supply of oxygen via the secondary air, and further oxidized to carbon dioxide throughout the remainder of reaction chamber 1. This avoids emission problems which occur when carbon monoxide is exhausted from the system.
Flue gas is discharged from reaction chamber 1 via discharge conduit 15 into particle separator 16. The particle separator 16 is typically a cyclone design which separates solids entrained in the flue gas discharged from reaction chamber 1, and recycles the separated solids via pressure seal 17 and recycle port 10 back to the lower portion of reaction chamber 1. It is important that the separated solids from particle separator 16 be recycled at a point below secondary air inlets 9. This assists in maintaining the low density of the dilute phase above the dense bed, i.e., a solids density approximately in the range between about 0.2 to 1.25 lb/ft.sup.3. It also increases the solids residence time which enhances the combustion efficiency of the system.
Optionally, at least one bed drain port 18 is disposed at the lower end of reaction chamber 1 to permit the removal of bed material, such as rocks, stones, used limestone, etc. Bed drain port 18 is connected to ash classifier 2 via bed drain conduit 19. Bed drain conduit 19 includes a control valve 20 which regulates the quantity of bed material removed at any given time. The bed material is then transferred to ash classifier 21 which separates fine particles from coarser fractions of the bed material, disposing of the coarser fraction via conduit 22 and returning the fine particles to reaction chamber 1 via conduit 23. Recycling of fines assists in maintaining the low solids density in the dilute phase and also increases the combustion efficiency of the system.
There are two regimes of fluidization in reaction chamber 1: (1) the lower dense bed where the coal, sorbent and recycled solids are mixed, and (2) an upper dilute phase where combustion is completed, sulphur products are absorbed, and heat is transferred to the water-cooled walls. The depth of the dense bed is usually 3 to 4 feet while the height of the dilute phase is usually 60 to 80 feet.
The two regimes are accomplished by bypassing some of the combustion air around the dense bed. The bypassed or secondary air enters above the dense bed at one or more levels. All levels of secondary air are usually introduced to the reaction chamber by ports arranged around the entire perimeter.
Despite the rapid development of fluid bed combustion technology, the problem of erosion of waterwall tubes and in-bed heat exchange tubes, as well as refractory-lined, tangent tube or metal plate walls, remains. The problem of erosion of in-bed heat exchange tubes was addressed in U.S. Pat. No. 4,714,049 (McCoy et al.), which issued on Dec. 22, 1987. This patent reduced or eliminated fluid bed in-bed tube erosion by increasing the fireside tube temperature by adding appropriately dimensioned longitudinal or circumferential fins to the in-bed heat exchange tubes in the reaction chamber.
Although U.S. Pat. No. 4,714,049 addressed erosion of in-bed heat exchange tubes, it did not contemplate the erosion problems associated with waterwall tubes, refractory bricks, tangent tubes or metal plates disposed about the perimeter of the reaction chamber. The problem of erosion of reaction chamber perimeter walls is documented in U.S. Pat. No. 5,005,528 (Virr), which issued on Apr. 9, 1991, and an article by Jason Makansi, "Special Report: Fluidized-Bed Boilers," Power, March 1991.
U.S. Pat. No. 5,005,528 suggests that one major disadvantage with conventional circulating fluidized bed boilers is severe erosion of the boiler's heat exchange tubes, especially those tubes which line the perimeter walls and roof of the combustor. The inventor thereof suggested that the erosion is caused by the high velocities necessary to achieve satisfactory heat transfer. It was observed that some tubes were away and fail after only 1,000 hours of operation, particularly those tubes located in the roof and corners of the reaction chamber. Various palatable methods have been proposed to combat erosion, such as, fins, metal spray, studs and refractory-linings. However, each of the aforementioned methods is extremely expensive and thus commercially undesirable. U.S. Pat. No. 5,005,528 overcame the waterwall tube erosion problem by means of a unique bubbling fluid bed boiler with recycle which incorporated the advantages of both the circulating fluid bed and bubbling fluid bed systems. This design, however, does not overcome the waterwall tube or other perimeter wall erosion prevalent in conventional circulating fluidized bed boilers designs.
The Makansi article suggests that increases in solids velocity to augment heat transfer and attain rated steam load caused erosion to become worse. Designs with lower velocities and/or low solids density experience generally less erosion. Makansi also pointed out that one type of erosion has been identified and classified as "sliding-ash" erosion. That is, particles flowing downward between water-cooled membrane tube walls of the combustor hit projections, such as, weld beads, and are deflected into the tubes. This results is an eventual failure of the waterwall tube. The current means for preventing sliding ash erosion is the removal of irregularities or abrupt changes in geometry by grinding, filling, or weld overlay.
Makansi identifies another area where erosion persists at the interface between the lower refractory-lined combustor bed area and the waterwall tubes. Several plants have installed small shelves to break up solids refluxing patterns. Another approach involved raising the height of the refractory level and applying a plasma spray coating to waterwalls on a three foot zone above the interface. However, some heat transfer capacity was lost. Still others have suggested that the basic interface design be modified by angling the waterwall tubes away from the furnace by bending the tubes in a serpentine manner directly above the interface to shield the interface from the solid particles.
This persistent problem of combustor or reaction chamber perimeter wall erosion is one of the largest deterrents associated with marketing and commercializing circulating fluidized bed boilers and reactors. A typical combustor perimeter wall construction for a circulating fluid bed boiler or reactor is shown in FIG. 2, attached hereto. The construction is commonly called "membrane" or "welded" wall where tubes 30 are welded together with longitudinal bars 32 between them. Between housing 34 is disposed insulation 36.
FIGS. 3a-3c clearly demonstrate known erosion points caused by downflowing solid particles impacting a vertically disposed waterwall tube. These erosion points require periodic repair and replacement which is not only costly in terms of maintenance, but also requires the shutting down of the boiler or reactor itself in order to perform such maintenance. Maintenance cost and service interruption are of great concern and constant investigation by boiler and reactor fabricators.
The present inventor has developed various unique reaction chamber or combustor configurations which substantially reduce or eliminate erosion of waterwall tubes or other types of perimeter walls used in fluidized bed boilers or reactor. The present invention to reduce or eliminate reaction chamber perimeter wall erosion applies equally to all perimeter wall construction, e.g., waterwall tube, metal plate, tangent tube, and refractory brick construction. It is particularly suited for reducing or eliminating erosion about reaction chamber perimeter walls having non-uniform geometry, protrusions, projections, etc. Some examples of which are: (1) tubes bent for openings such as the coal feed pipes or observation ports, (2) weld projections where tubes are welded together, and (3) the interface between refractory bricks and the perimeter wall.
The present invention also provides many additional advantages which shall become apparent as described below.